Method and apparatus for transmitting data between wireless devices in wireless communication system

ABSTRACT

A method for transmitting data between wireless devices in a wireless communication system is provided. A first wireless device acquires downlink reception timing with a base station. The first wireless device determines transmission timing for direct communication with a second wireless device based on the downlink reception timing with the base station. The first wireless device transmits data for the direct communication to the second wireless device at the transmission timing for the direct communication.

CROSS-REFERENCE TO RELATED APPLICATIONS

Pursuant to 35 U.S.C. §119(e), this application claims the benefit ofU.S. Provisional Patent Application Ser. No. 61/601,559 filed on Feb.22, 2012, and U.S. Provisional Patent Application Ser. No. 61/602,570,filed on Feb. 23, 2012, the contents of which are hereby incorporated byreference herein in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to wireless communication. Moreparticularly, the present invention relates to a method for transmittingdata between wireless devices in a wireless communication system and anapparatus using the same.

2. Discussion of the Related Art

The next generation wireless communication system under active researchaims a system capable of transmitting various kinds of information suchas video and wireless data, being evolved from the initial systemproviding voice-oriented services. The fourth-generation wirelesscommunication currently under development subsequent to thethird-generation wireless communication aims to support high speed datatransmission with 1 Gbps (gigabits per second) data rate in the downlinkand 500 Mbps (megabits per second) in the uplink. The main objective ofwireless communication system is to provide a plurality of users withreliable communication means independent of their location and mobility.However, any wireless communication channel always reveals non-idealcharacteristics such as path loss, noise, fading due to multipath,inter-symbol interference (ISI), or Doppler Effect due to mobility of aterminal Various technologies are under development to overcomenon-ideal characteristics of wireless communication channels and improvereliability thereof.

Meanwhile, data capacity for cellular wireless systems is everincreasing according to the introduction of machine type communication(MTC) and the advent and deployment of various devices such as smartphones and tablet PCs. Various technologies are under development tomeet the needs for high data capacity. For example, carrier aggregation(CA) technology and cognitive radio (CR) technology are good examples ofan effort to utilize frequency bandwidth more efficiently. Also,multi-antenna technology, multi-base station collaboration technology, adirect communication system, etc. to increase data capacity withinlimited frequency bandwidth are being studied.

In direct communication system, user equipments (UEs) directly performtransmission and reception between themselves without relay of a basestation. In a direct communication system, there is needed a method foracquiring transmission/reception timing and/or synchronization betweenthe UEs for performing transmission and reception between the UEs, and amethod for minimizing the interference that can occur during directcommunication.

SUMMARY OF THE INVENTION

The present invention provides a method for data transmission betweenwireless devices in wireless communication system.

The present invention also provides a method for acquiring transmissionand/or reception timing in direct communication and apparatus using thesame.

The present invention also provides a method for acquiringsynchronization between wireless devices in direct communication andapparatus using the same.

The present invention also provides a method for reducing interferencethat can occur in direct communication and apparatus using the same.

In an aspect, a method for transmitting data between wireless devices ina wireless communication system is provided. The method comprises:acquiring, by a first wireless device, downlink reception timing with abase station, determining, by the first wireless device, transmissiontiming for direct communication with a second wireless device based onthe downlink reception timing with the base station, and transmitting,by the first wireless device, data for the direct communication to thesecond wireless device at the transmission timing for the directcommunication.

The data for the direct communication may be transmitted via a subframeon an uplink resource used for a communication with the base station.

The data for the direct communication may be transmitted via a subframeon a downlink resource used for a communication with the base station.

The subframe on the downlink resource may include at least one guardsymbol.

The subframe on the downlink resource may include at least one puncturedsymbol.

The subframe on the downlink resource may use extended cyclic prefix(CP).

In another aspect, a method for transmitting data between wirelessdevices in a wireless communication system is provided. The methodcomprises: acquiring, by a first wireless device, uplink transmissiontiming with a base station; determining, by the first wireless device,transmission timing for direct communication with a second wirelessdevice based on the uplink transmission timing with the base station;and transmitting, by the first wireless device, data for the directcommunication to the second wireless device at the transmission timingfor the direct communication.

In another aspect, a wireless device in a wireless communication systemis provided. The wireless device comprises: a RF (Radio Frequency) unittransmitting and receiving radio signals, and a processor connected tothe RF unit. The processor is configured to acquire downlink receptiontiming with a base station, determine transmission timing for directcommunication with a neighbor wireless device based on the downlinkreception timing with the base station, and transmit data for the directcommunication to the neighbor wireless device at the transmission timingfor the direct communication.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a structure of a downlink radio frame in 3GPP LTE-A.

FIG. 2 illustrates an example of a resource grid for one downlink slot.

FIG. 3 illustrates a structure of a downlink subframe.

FIG. 4 illustrates a structure of an uplink subframe.

FIGS. 5 to 7 illustrate one example of an RB on which a CRS is mapped.

FIG. 8 is one example of an RB to which a DM-RS is mapped.

FIG. 9 is one example of an RB to which a CSI-RS is mapped.

FIG. 10 illustrates a conventional wireless communication system and adirect communication system.

FIG. 11 is a flow diagram illustrating one example of directcommunication between user equipments (UEs).

FIG. 12 illustrates an example of acquiring DL reception timing and ULtransmission timing in general wireless communication.

FIG. 13 illustrates the DL reception timing and the UL transmissiontiming of each entity in the example of FIG. 12.

FIG. 14 illustrates a method for acquiring d-DL (direct-DL) timing andd-UL (direct-UL) timing according to the one example of the presentinvention.

FIG. 15 illustrates a method for acquiring d-DL timing and d-UL timingaccording to another example of the present invention.

FIG. 16 illustrates a method for acquiring d-DL timing and d-UL timingaccording to another example of the present invention.

FIG. 17 illustrates a method for acquiring d-DL timing and d-UL timingaccording to another example of the present invention.

FIGS. 18-21 illustrate interferences that can occur in the examples ofFIGS. 14-17.

FIGS. 22-24 illustrate a method for using a guard symbol according toone example of the present invention.

FIG. 25 illustrates one example of implicitly puncturing the last symbolof a sounding subframe.

FIG. 26 illustrates one example of a sounding subframe for directcommunication.

FIG. 27 illustrates one example of using a subframe with an extended CPfor direct communication.

FIG. 28 illustrates one example of a configuration of a subset of MBSFNsubframes as a subframe for direct communication.

FIG. 29 illustrates one example of applying time offset to a subframefor direct communication.

FIG. 30 illustrates a method for data transmission according to oneexample of the present invention.

FIG. 31 illustrates a wireless communication system in which anembodiment of the present invention is implemented.

DETAILED DESCRIPTION OF THE INVENTION

The technology described below can be used for various multiple accessschemes including CDMA (Code Division Multiple Access), FDMA (FrequencyDivision Multiple Access), TDMA (Time Division Multiple Access), OFDMA(Orthogonal Frequency Division Multiple Access) and SC-FDMA (SingleCarrier-Frequency Division Multiple Access). CDMA can be implemented byusing such radio technology as UTRA (Universal Terrestrial Radio Access)or CDMA2000. TDMA can be implemented by using such radio technology asGSM (Global System for Mobile communications)/GPRS (General Packet RadioService)/EDGE (Enhanced Data Rates for GSM Evolution). OFDMA can berealized by using such radio technology as the IEEE 802.11 (Wi-Fi), IEEE802.16 (WiMAX), IEEE 802.20, and E-UTRA (Evolved UTRA). UTRA is part ofspecifications for UMTS (Universal Mobile Telecommunications System).The 3GPP LTE is part of E-UMTS (Evolved UMTS) using E-UTRA, which usesOFDMA radio access for the downlink and SC-FDMA on the uplink. The LTE-A(Advanced) is an evolved version of the LTE.

A user equipment (UE) may be fixed or mobile and called in differentterms such as a wireless device, a mobile station (MS), a user terminal(UT), a subscriber station (SS), a personal digital assistant (PDA), awireless modem, or a handheld device.

A base station (BS) usually refers to a fixed station communicating witha UE, which is called in different terms such as an evolved-NodeB (eNB),a base transceiver system (BTS), or an access point (AP).

In what follows, the downlink (DL) refers to a communication link from aBS to a UE while the uplink (UL) from the UE to the BS. In the DL, atransmitter may be a part of the BS while a receiver a part of the UE.In the UL, a transmitter may be a part of the UE while a receiver partof the BS.

In the description below, application of the present invention isdescribed with reference to 3GPP LTE based on 3GPP TS (TechnicalSpecification) release 8, or 3GPP LTE-A based on 3GPP TS release 10. Theexamples in the specification are only intended to illustrate thepresent invention and should not be understood to limit the invention,and the present invention can be applied to various wirelesscommunication networks. In the following description, LTE refers to thewireless system including LTE and/or LTE-A.

FIG. 1 illustrates a structure of a downlink radio frame in 3GPP LTE-A.The section 6 of the 3GPP TS 36.211 V10.4.0 (2012-12) “Evolved UniversalTerrestrial Radio Access (E-UTRA); Physical Channels and Modulation(Release 10)” may be incorporated herein by reference.

A radio frame consists of 10 subframes indexed with 0 to 9. One subframeconsists of two consecutive slots. A time required for transmitting onesubframe is defined as a transmission time interval (TTI). For example,one subframe may have a length of 1 ms and one slot may have a length of0.5 ms.

One slot may include a plurality of orthogonal frequency divisionmultiplexing (OFDM) symbols in a time domain. Since the 3GPP LTE usesorthogonal frequency division multiple access (OFDMA) in a downlinkmultiple access scheme, the OFDM symbol is only for expressing onesymbol period in the time domain, and there is no limitation in amultiple access scheme or terminologies. For example, the OFDM symbolmay also be called in different terms such as a single carrier frequencydivision multiple access (SC-FDMA) symbol when SC-FDMA is used in theuplink multiple access scheme. A resource block (RB) includes multipleconsecutive subcarriers at one slot in the unit of resource allocation.

The example of the structure of a wireless frame in FIG. 1 is just oneexample. Therefore, the number of subframes included in the wirelessframe, the number of slots included in the subframe, or the number ofOFDM symbols included in the slot can be variously determined. 3GPP LTEdefines that one slot includes 7 OFDM symbols in normal cyclic prefix(CP), and one slot includes 6 OFDM symbols in extended CP.

FIG. 2 illustrates an example of a resource grid for one downlink slot.

The downlink slot includes multiple OFDM symbols in time domain, andN_(RB) resource blocks in frequency domain. The number of resourceblocks included in the downlink slot, N_(RB), depends on the downlinktransmission bandwidth configured at the cell. In LTE system, forexample, N_(RB) can be one from 6 to 110. One resource block includesmultiple subcarriers in frequency domain. The structure of the uplinkslot can be the same as that of the downlink slot.

Each element on the resource grid is called a resource element (RE). Theelement on the resource grid can be identified by the index pair (k, 1)in the slot. Here, k (k=0, . . . , N_(RB)×12−1) is the subcarrier indexin frequency domain, and 1 (1=0, . . . , 6) the OFDM symbol index intime domain.

Although one resource block is described to include 7×12 resourceelement composed of 7 OFDM symbols in time domain and 12 subcarriers infrequency domain in this specification, the example is for the purposeof illustration only and is not intended to limit the number of OFDMsymbols and subcarriers in the resource block. The number of OFDMsymbols and subcarriers can be variously modified depending on thelength of CP, frequency spacing, etc.

FIG. 3 illustrates a structure of a downlink subframe.

DL (downlink) subframe is divided into a control region and a dataregion in time domain. The control region includes maximum of 4preceding OFDM symbols of the first slot in the subframe, though thenumber of OFDM symbols included in the control region can be changed. Inthe control region, Physical Downlink Control Channel (PDCCH) and othercontrol channels are allocated, and in the data region, PhysicalDownlink Shared Channel (PDSCH) is allocated.

As disclosed in the 3GPP TS 36.211 V10.4.0, the 3GPP LTE/LTE-A defines aphysical channel, including a PDCCH, a Physical Control Format IndicatorChannel (PCFICH), and a Physical Hybrid-ARQ Indicator Channel (PHICH).Also, control signals transmitted from a physical layer include aPrimary Synchronization Signal (PSS), a Secondary Synchronization Signal(SSS), and a random access preamble.

The PSS is carried by the last OFDM symbol of a first slot (first slotof a first subframe (subframe with index 0)) and the 11th slot (firstslot of a sixth subframe (subframe with index 5)). The PSS is used forobtaining OFDM symbol synchronization or slot synchronization, andassociated with a physical cell identify (ID). A Primary SynchronizationCode (PSC) is a sequence used for the PSS and the 3GPP LTE defines threePSCs. According to the cell ID, one from among the three PSCs istransmitted to the PSS. The same PSC is used for each of the last OFDMsymbols of the first and the 11^(th) slot.

The SSS is divided into a first and a second SSS. The first and thesecond SSS are carried by an OFDM symbol adjacent to the OFDM symbolcarrying the PSS. The SSS is used for obtaining frame synchronization.The SSS is used for obtaining cell ID together with the PSS. The firstand the second SSS use Secondary Synchronization Codes (SSCs) differentfrom each other. In case the first and the second SSS carry 31sub-carriers respectively, two SSC sequences of length 31 are used forthe first and the second SSS, respectively.

The PCFICH transmitted in the first OFDM symbol of a subframe carriescontrol format indicator (CFI) which indicates the number of OFDMsymbols (namely, size of the control region) used for carrying controlchannels within a subframe. The UE first receives the CFI through thePCFICH and monitors the PDCCH. The PCFICH does not use blind decodingbut transmitted through the fixed PCFICH resources of a subframe.

The PDCCH carries control information which is called downlink controlinformation (DCI). DCI may include resource allocation of PDSCH (whichis also called DL grant), resource allocation of PUSCH (which is calledUL grant), and activation of a set of transmission power controlcommands for individual UEs within a UE group and/or voice over internetprotocol (VoIP).

The PHICH carries ACK (positive acknowledgement)/NACK (negativeacknowledgement) signal for UL hybrid automatic repeat request (HARQ).The ACK/NACK signal about the UL data on the PUSCH transmitted by the UEis transmitted through the PHICH.

FIG. 4 illustrates a structure of an uplink subframe.

The uplink subframe can be divided into a control region and a dataregion in frequency domain. To the control region is allocated PhysicalUplink Control Channel (PUCCH) for uplink control information to betransmitted. To the data region is allocated Physical Uplink SharedChannel (PUSCH) for data to be transmitted.

PUCCH for one terminal is allocated to the resource block pair (RB pair)at the subframe. The resource blocks belonging to the resource blockpair occupy subcarriers which are different each other at the first andsecond slots. The frequency occupied by the resource blocks belonging tothe resource block pair allocated to PUCCH is changed based on the slotboundary. In this process, it is that RB pair allocated to PUCCH isfrequency-hopped at the slot boundary. By transmitting the uplinkcontrol information through different subcarriers according to the timeby the terminal, frequency diversity gain can be obtained. The locationindex, m, represents the logical frequency domain location of theresource block pair allocated to PUCCH at the subframe.

The uplink control information transmitted on PUCCH includes HARQACK/NACK, channel quality indicator (CQI) representing downlink channelstate, and scheduling request (SR) which is an uplink wireless resourceallocation request.

Now, a reference signal will be described.

A reference signal (RS) is usually transmitted in the form of asequence. A reference signal sequence may employ a random sequencewithout being limited by particular conditions. The reference signalsequence may employ a PSK (Phase Shift Keying)-based computer generatedsequence. Examples of the PSK include binary phase shift keying (BPSK),quadrature phase shift keying (QPSK), and so on. Similarly, thereference signal sequence may employ constant amplitude zeroauto-correlation (CAZAC) sequence. Examples of the CAZAC sequenceinclude Zadoff-Chu (ZC)-based sequence, ZC sequence with cyclicextension, ZC sequence with truncation, and so on. Meanwhile, thereference signal sequence may employ a pseudo-random (PN) sequence.Examples of the PN sequence include m-sequence, computer-generatedsequence, gold sequence, Kasami sequence, and so on. Also, the referencesignal sequence may employ a cyclically shifted sequence.

A DL reference signal can be classified into a cell-specific RS (CRS),multimedia broadcast and multicast single frequency network (MBSFN) RS,UE-specific RS, positioning RS (PRS), and channel state information(CSI) RS. The CRS is an RS transmitted to all the UEs within a cell,which can be used for channel measurement about CQI feedback and channelestimation about the PDSCH. The MBSFN RS can be transmitted from asubframe allocated for MBSFN transmission. The UE-specific RS is an RSreceived by a particular UE or a particular UE group within a cell,which may be called a demodulation RS (DM-RS). The DM-RS is mostly usedfor a particular UE or a particular UE group to perform datademodulation. The PRS may be used for position estimation of the UE.CSI-RS is used for channel estimation for the PDSCH of the LTE-A UE. TheCSI-RS is disposed in a relatively sparse fashion in the spectral ortemporal region, and can be punctured in a general subframe or dataregion of the MBSFN subframe. In case of need for estimating CSI, the UEmay report CQI, PMI, RI, etc.

The CRS is transmitted from all the DL subframes within a cellsupporting PDSCH transmission. The CRS can be transmitted through theantenna port 0 to 3, and the CRS may be defined only for Δf=15 kHz. TheCSI-RS may refer to the Section 6.10.1 of the 3GPP TS 36.211 V10.4.0(2012-12).

FIGS. 5 to 7 illustrate one example of an RB on which a CRS is mapped.

FIG. 5 is one example of a pattern in which the CRS is mapped onto theRB in case the BS uses a single antenna port; FIG. 6 for the case wherethe BS uses two antenna ports; FIG. 7 for the case where the BS usesfour antenna ports. Also, the CRS pattern may be used for supporting thecharacteristics of the LTE-A. For example, the CRS pattern may be usedfor supporting the characteristics such as coordinated multi-point(CoMP) transmission and reception method; or spatial multiplexing. Also,the CRS may be used for channel quality measurement, CP detection,time/frequency synchronization, etc.

With reference to FIGS. 5 to 7, in case of multi-antenna transmissionwhere the BS uses a plurality of antenna ports, one resource grid isallocated to each antenna port. ‘R0’ denotes a RS for a first antennaport; ‘R1” for a second antenna port; ‘R2’ for a third antenna port; and‘R3’ for a fourth antenna port. Positions of R0 to R3 within a subframedo not overlap with each other. l is the position of the OFDM symbolwithin a slot, whose value ranges from 0 to 6 for a normal CP. The RSfor each antenna port in one OFDM symbol is placed at interval of sixsub-carriers. The number of R0 within the subframe is the same as thenumber of R1 and the number of R2 and the number of R3 are the same toeach other. The number of R2 and R2 within the sub-frame is less thanthe number of R0 and R1. A resource element used for the RS of oneantenna port is not used as an RS for another antenna, which is intendednot to cause interference between antenna ports.

CRSs as many as the number of antenna ports are always transmittedirrespective of the number of streams. The CRS has an independentreference signal for each antenna port. The position of the CRS in thefrequency and time domain within a subframe is determined independentlyof the UE. Also, a CRS sequence multiplied to the CRS is generatedindependently of the UE. Therefore, all the UEs within a cell canreceive the CRS. However, the position of the CRS within the subframeand the CRS sequence may be determined according to the cell ID. Theposition of the CRS in the time domain within the subframe may bedetermined according to the antenna port number and the number of OFDMsymbols within the resource block. The position of the CRS in thefrequency domain within the subframe may be determined according to theantenna number, cell ID, OFDM symbol index (l), slot number within aradio frame, and so on.

The CRS sequence may be applied in units of OFDM symbols within onesubframe. The CRS sequence may vary depending on the slot number withinone radio frame, OFDM symbol index within the slot, type of CP, and soon. The number of RS subcarriers for each antenna port in one OFDMsymbol is 2. If it is assumed that a subframe contains N_(RB) resourceblocks in the frequency domain, the number of RS subcarriers for eachantenna in one OFDM symbol becomes 2×N_(RB). Therefore, length of theCRS sequence becomes 2×N_(RB).

Equation 1 represents one example of a CRS sequence r(m)

$\begin{matrix}{{{r(m)} = {{\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {2m} \right)}}} \right)} + {j\; \frac{1}{\sqrt{2}}\left( {1 - {2 \cdot \left( {{2m} + 1} \right)}} \right)}}},} & {\langle{{Equation}\mspace{14mu} 1}\rangle}\end{matrix}$

where m is 0, 1, . . . , 2N_(RB) ^(max)−1. 2N_(RB) ^(max) is the numberof resource blocks corresponding to the maximum bandwidth. For example,2N_(RB) ^(max) is 110 in the 3GPP LTE. c(i) is a PN sequence, which is asimulated, random sequence, and can be defined by a gold sequence oflength −31. Equation 2 represents one example of the gold sequence c(n).

c(n)=(x ₁(n+N _(C))+x ₂(n+N _(C)))mod 2

x ₁(n+31)=(x ₁(n+3)+x ₁(n))mod 2

x ₂(n+31)=(x ₂(n+3)+x ₂(n+2)+x ₂(n+1)+x ₂(n))mod 2  <Equation 2>

where Nc=1600; x₁(i) is a first m-sequence; x₂(i) is a secondm-sequence. For example, the first and the second m-sequence may beinitialized for each OFDM symbol according to cell ID, slot numberwithin one radio frame, OFDM symbol index within the slot, type of CP,and so on.

In case of a system having bandwidth less than 2N_(RB) ^(max), only apredetermined part with length of 2×N_(RB) may be selected and used fromthe RS sequence with length of 2×2N_(RB) ^(max).

Frequency hopping may be applied to the CRS. A frequency hopping patternmay take one radio frame (10 ms) for its period and each frequencyhopping pattern corresponds to one cell ID group.

The DM-RS is provided for the PDSCH transmission and is transmitted onthe antenna port p=5, p=7, 8, or p=7, 8, . . . , v+6. At this time, vdenotes the number of layers used for the PDSCH transmission. The DM-RSis transmitted to one UE through any one of antenna ports belonging to aset S, where S={7, 8, 11, 13} or S={9, 10, 12, 14}. The DM-RS exists andis valid for demodulation of the PDSCH only when transmission of thePDSCH is associated with the corresponding antenna port. The DM-RS istransmitted only at the RBs to which the corresponding PDSCH is mapped.The DM-RS is not transmitted at resource elements through which either aphysical channel or a physical signal is transmitted, irrespective ofantenna ports. The DM-RS may refer to the Section 6.10.3 of the 3GPP TS36.211 V10.4.0 (2012-12).

FIG. 8 is one example of an RB to which a DM-RS is mapped.

FIG. 8 illustrates resource elements used for the DM-RS in the normal CPstructure. Rp denotes a resource element used for DM-RS transmissionthrough the antenna port p. For example, R5 denotes a resource elementto which the DM-RS for the antenna port 5 is transmitted. Also, withreference to FIG. 8, the DM-RS for the antenna port 7 and 8 istransmitted through the resource element corresponding to a first,sixth, and eleventh subcarrier (subcarrier index 0, 5, 10) of a sixthand seventh OFDM symbol (OFDM symbol index 5, 6) of each slot. The DM-RSfor the antenna port 7 and 8 can be distinguished by an orthogonalsequence of length 2. The DM-RS for the antenna port 9 and 10 istransmitted through the resource element corresponding to a second,seventh, and twelfth subcarrier (subcarrier index 1, 6, 11) of a sixthand seventh OFDM symbol (OFDM symbol index 5, 6) of each slot. The DM-RSfor the antenna port 9 and 10 can be distinguished by an orthogonalsequence of length 2. Also, since S={7, 8, 11, 13} or S={9, 10, 12, 14},the DM-RS for the antenna port 11 and 13 is mapped to the resourceelement to which the DM-RS for the antenna port 7 and 8 is mapped whilethe DM-RS for the antenna port 12 and 14 is mapped to the resourceelement to which the DM-RS for the antenna port 9 and 10 is mapped.

The CSI-RS is transmitted through 1, 2, 4, or 8 antenna ports. Theantenna ports used for this case correspond to p=15, p=15, 16, p=15, . .. , 18, and p=15, . . . , 22. The CSI-RS can be defined only for Δf=15kHz. The CSI-RS may refer to the Section 6.10.3 of the 3GPP TS 36.211V10.1.0 (2012-12).

For transmission of the CSI-RS, up to 32 different configurations fromeach other can be employed to reduce inter-cell interference (ICI) in amulti-cell environment as well as a heterogeneous network environment.Configurations for the CSI-RS differ from each other according to thenumber of antenna ports and CP within a cell; and adjacent cells mayassume configurations different from each other as possibly as can be.Also, the CSI-RS configurations can be divided into the cases of beingapplied to both frequency division duplex (FDD) and time division duplex(TDD) frame and only to TDD frame according to frame structure. Aplurality of CSI-RS configurations may be used for a single cell. Forthe UE assuming non-zero transmission power may employ 0 or 1 CSIconfiguration while the UE assuming zero transmission power may employ 0or several CSI configurations. The UE does not transmit the CSI-RS for aspecial subframe of TDD frame; a subframe where transmission of theCSI-RS collides with a synchronization signal, a physical broadcastchannel (PBCH), and system information block type 1; or a subframe towhich a paging message is transmitted. In addition, in a set S whereS={15}, S={15, 16}, S={17, 18}, S={19, 20} or S={21, 22}, the resourceelement to which the CSI-RS for one antenna port is transmitted is notused for transmission of the CSI-RS for the PDSCH or another antennaport.

FIG. 9 is one example of an RB to which a CSI-RS is mapped.

FIG. 9 illustrates resource elements used for the CSI-RS in the normalCP structure. Rp denotes a resource element used for CSI-RS transmissionthrough the antenna port p. With reference to FIG. 9, the CSI-RS for theantenna port 15 and 16 is transmitted through the resource elementcorresponding to a third subcarrier (subcarrier index 2) of a sixth andseventh OFDM symbol (OFDM symbol index 5, 6) of a first slot. The CSI-RSfor the antenna port 17 and 18 is transmitted through the resourceelement corresponding to a ninth subcarrier (subcarrier index 8) of asixth and seventh OFDM symbol (OFDM symbol index 5, 6) of a first slot.The CSI-RS for the antenna port 19 and 20 is transmitted through thesame resource element to which the CSI-RS for the antenna port 15 and 16is transmitted while the CSI-RS for the antenna port 21 and 22 istransmitted through the same resource element to which the CSI-RS forthe 17 and 18 is transmitted.

Meanwhile, direct communication means a communication method forperforming transmission and reception between UEs without the relay of aBS, directly. FIG. 10 illustrates a conventional wireless communicationsystem and a direct communication system.

Referring to FIG. 10, UEs in a conventional communication system alwayscommunicate with a BS. On the other hand, the UE in a directcommunication system can communicate with other UE in addition to a BS.Direct communication is preferably carried out when UEs are ingeographically neighboring region or channel state between UEs is good.When direct communication is carried out, controls on UEs are stillperformed by the BS (dotted line). On the other hand, actual data orinformation related to the actual data (e.g. HARQ, management/controlinformation of direct communication network) is communicated throughdirect communication between UEs (solid line).

The BS instructs UEs to perform direct communication for establishinglink for UEs which want direct communication, and determines resourcesfor the direct communication. The BS informs all UE or a primary UE,which the BS determines from all the UEs, of the resources allocated fordirect communication. Then, UEs can perform direct transmission andreception of data under the control of the BS but without relay of theBS.

Although all data can be transmitted and received through direct linkbetween UEs, as described in the above, it is preferable to transmit andreceive only actual data and minimal control information related to thedata between UEs, and to transmit and receive other control informationthrough the BS. In other words, connection and communication with the BSare not excluded even when direct communication between UEs isperformed. For example, direct communication request/responseinformation, scheduling information such as resource allocationinformation, security information, and other information needed fordirect communication can be transmitted and received through the BS.

FIG. 11 is a flow diagram illustrating one example of directcommunication between user equipments (UEs).

First, UE 1 requests direct communication with UE 2 to the BS (S1110).The request can be made by the BS first, or UE 1 can directly request toUE 2. Also, direct communication can be started without request on acontention basis.

The BS allocates downlink and/or uplink resource for directcommunication between UE 1 and UE 2 (S1120). At this step, resourceallocation for UE 1 and UE 2 can be signaled to each UE independently orcommonly signaled.

Before the step S1120, the step that the BS queries UE 2 on theinitiation of direct communication and responses the result to UE 1 canbe added.

Then, UE 1 and UE 2 perform direct communication by using allocatedresource (S1130).

The steps in FIG. 11 are represented for the convenience of description,and the order of the steps can be changed depending on the scheduling ofthe BS and the state of each UE, and additional control signals andmeasurement signals can be transmitted and received between the BS andUEs.

In the description below, the method for acquiring transmission andreception timing, and/or synchronization between UEs in this directcommunication system will be described. In the description of thespecification, timing can mean the transmission timing for transmittinga transmission signal, the timing for determining subframe boundary of atransmission signal, the reception timing for detecting a receptionsignal, or the time for determining subframe boundary of a receptionsignal.

Also, for the convenience of explanation, the application of the presentinvention is described based on FDD direct communication system. Thisapplication, however, is for the purpose of illustration only, and thepresent invention can also be applied to TDD direct communicationsystem.

Also, for the convenience of explanation, it is assumed that the DLsubframe boundary and UL subrame boundary of the BS are aligned based onabsolute time. This assumption, however, is for the purpose ofillustration only, and the present invention can be applied to the casewhere the DL subframe boundary and UL subframe boundary are differenteach other.

In conventional wireless communication system, UE acquires DL receptiontiming by using DL synchronization signal of the BS. Then, initialtransmission timing for UL is set from the acquired DL subframeboundary, and UL transmission timing is acquired through random access.

FIG. 12 illustrates an example of acquiring DL reception timing and ULtransmission timing in general wireless communication.

Referring to FIG. 12, the BS transmits DL signal to UE 1. UE 1 receivesthe DL signal with DL propagation delay according to the distance fromthe BS. UE 1 can acquire DL reception timing based on the DLsynchronization signal.

Then, UE 1 performs initial random access procedure to acquire uplinktransmission timing. UE 1, assuming that UL transmission timing is thesame as DL reception timing, performs Physical Random Access Channel(PRACH) transmission. At this step, the offset between DL receptiontiming and UL transmission timing can be defined in advance. In otherwords, system can be configured so that UL subframe boundary isdistanced from DL subframe boundary by predetermined offset.

Then, the BS receives random access preamble with UL propagation delayaccording to the distance from the UE 1. Therefore, the BS receivesrandom access preamble with the delay corresponding to the sum of DLpropagation delay and UL propagation delay. At this step, the BSestimates the total delay by PRACH detection to instruct UL transmissiontiming to the UE 1. This is called ‘timing advance (TA)’.

The same procedure can be performed for UE 2. Referring to FIG. 12, UE 2is assumed to be located at the location which is relatively far fromthe BS compared to UE 1. In other words, DL propagation delay and ULpropagation delay of UE 2 is larger than those of UE 1. TA_(diff)represents the difference between UL transmission timings of the twoUEs.

FIG. 13 illustrates the DL reception timing and the UL transmissiontiming of each entity in the example of FIG. 12.

In the example of FIG. 12, as described above, UE 2, which is locatedrelatively far from the BS compared to UE 1, has larger DL propagationdelay and UL propagation delay. Therefore, referring to FIG. 13, the DLreception timing of UE 2 is located behind the DL reception timing of UE1, and the UL transmission timing of UE 2, on the other hand, is locatedahead of transmission timing of UE 1.

The method for configuring/acquiring d-DL (direct-DL) and d-UL(direct-UL) timing for direct communication based on the DL and ULtiming which the UE has acquired through the process described abovewill be described below. In the below, unless specifically mentioned, DLand UL mean conventional downlink and uplink of communication with theBS, and d-DL and d-UL mean downlink and uplink of direct communicationbetween UEs. The d-DL refers to a reception link from one UE to anotherUE, and the d-UL refers to a transmission link from one UE to anotherUE. Also, for the convenience of description, d-DL reception and d-ULtransmission will be explained in the unit of subframe. Thisexplanation, however, is for the purpose of illustration only, and d-DLreception and d-UL transmission can be performed in the unit of slot orOFDMA symbol (or SC-FDMA symbol, discrete Fourier transform spread(DFT-S) OFDMA symbol).

FIG. 14 illustrates a method for acquiring d-DL (direct-DL) timing andd-UL (direct-UL) timing according to the one example of the presentinvention.

In FIG. 14, T_(diff-DL) and T_(diff-UL) represent the differences of DLreception timings and UL transmission timings between UE 1 and UE 2,respectively. Theoretically, since DL timing is associated withunidirectional propagation delay and UL timing is associated withbidirectional propagation delay, T_(diff-UL) has two times the valuecompared to the value of T_(diff-DL). Assuming two UEs are located inthe same line connecting the BS, the unidirectional propagation delayT_(diff-UE) according to the distance between two UEs can be representedas in Equation 3.

T _(diff-UE) =T _(diff-DL) =T _(diff-UL)/2  <Equation 3>

When two UEs are located with a distance of about 1 km, the propagationdelay T_(diff-UE) is 3.3356 us, and when two UEs are located nearly witha short distance (e.g. device-to-device (D2D) coverage), the propagationdelay T_(diff-UE) can be ignored.

Referring to FIG. 14, UE 1 and UE 2 perform d-UL transmission and d-DLreception using DL resource based on the UL transmission timing. The ULtransmission timing can be configured by the BS or acquired from the BS.

UE 1 performs d-UL transmission 1 at the first DL subframe based on theUL timing 1, and UE 2 performs d-UL transmission 2 at the second DLsubframe based on the UL timing 2. At this step, predetermined offsetcan be applied to the UL timing, or d-UL transmission can be performedat part of the subframe (e.g. slot) rather than at the entire subframe.

UE 2 performs d-DL reception 1 at the first DL subframe with time delayof T_(diff-UE). UE 1 also performs d-DL reception 2 at the second DLsubframe with time delay of T_(diff-UE). The data received by d-DLreception 1 is the data transmitted by d-UL transmission 1, and the datareceived by d-DL reception 2 is the data transmitted by d-ULtransmission 2. The d-DL reception timing 1 and 2 do not coincide withthe DL reception timing 1 and 2, but lags behind the DL reception timing1 and 2, respectively. Therefore, different timing than conventional DLtiming is required for the d-DL reception.

Meanwhile, it is preferable that the DL subframe used for directcommunication does not include the DL data received from the BS. Forthis purpose, when direct communication is performed, the BS can emptycorresponding DL subframe to allocate the DL subframe as a dedicatedresource for direct communication.

According to the method described above, UE can perform d-DL receptionusing the same frequency region as DL reception without additionalhardware. At this step, the UE stores d-DL timing other thanconventional DL timing in the buffer and can enhance the accuracy of thed-DL timing through the process of detecting and/or demodulation.

FIG. 15 illustrates a method for acquiring d-DL timing and d-UL timingaccording to another example of the present invention.

The meanings of T_(diff-DL), T_(diff-UL) and T_(diff-UE) in FIG. 15 arethe same as those in FIG. 14.

Referring to FIG. 15, UE 1 and UE 2 perform d-UL transmission and d-DLreception using UL resource based on the DL reception timing. The DLreception timing can be configured by the BS or acquired from the BS.

UE 1 performs d-UL transmission 1 at the first UL subframe based on theDL timing 1, and UE 2 performs d-UL transmission 2 at the second ULsubframe based on the DL timing 2. At this step, predetermined offsetcan be applied to the DL timing, or d-UL transmission can be performedat part of the subframe (e.g. slot) rather than at the entire subframe.

UE 2 performs d-DL reception 1 at the first UL subframe with time delayof T_(diff-UE). UE 1 also performs d-DL reception 2 at the second ULsubframe with time delay of T_(diff-UE). The data received by d-DLreception 1 is the data transmitted by d-UL transmission 1, and the datareceived by d-DL reception 2 is the data transmitted by d-ULtransmission 2. The d-DL reception timing 1 coincides with the DLreception timing 1, but the d-DL reception time does not coincide withthe DL reception timing 2. Therefore, different timing than conventionalDL timing 2 is required for the d-DL reception 2.

Meanwhile, it is preferable that the UL subframe used for directcommunication does not include the UL data transmitted to the BS. Forthis purpose, when direct communication is performed, the BS can emptycorresponding UL subframe to allocate the UL subframe as a dedicatedresource for direct communication.

According to the method described above, UE can perform d-ULtransmission using the same frequency region as UL transmission withoutimplementation of additional hardware for d-UL transmission.

FIG. 16 illustrates a method for acquiring d-DL timing and d-UL timingaccording to another example of the present invention.

The meanings of T_(diff-DL), T_(diff-UL) and T_(diff-UE) in FIG. 16 arethe same as those in FIG. 14.

Referring to FIG. 16, UE 1 and UE 2 perform d-UL transmission and d-DLreception using DL resource based on the DL reception timing. The DLreception timing can be configured by the BS or acquired from the BS.

UE 1 performs d-UL transmission 1 at the first DL subframe based on theDL timing 1, and UE 2 performs d-UL transmission 2 at the second DLsubframe based on the DL timing 2. At this step, predetermined offsetcan be applied to the DL timing, and d-UL transmission can be performedat part of the subframe (e.g. slot) rather than at the entire subframe.

UE 2 performs d-DL reception 1 at the first DL subframe with time delayof T_(diff-UE). UE 1 also performs d-DL reception 2 at the second DLsubframe with time delay of T_(diff-UE). The data received by d-DLreception 1 is the data transmitted by d-UL transmission 1, and the datareceived by d-DL reception 2 is the data transmitted by d-ULtransmission 2. The d-DL reception timing 1 coincides with the DLreception timing 1, but the d-DL reception time does not coincide withthe DL reception timing 2. Therefore, different timing than conventionalDL timing 2 is required for the d-DL reception 2.

Meanwhile, it is preferable that the DL subframe used for directcommunication does not include the DL data received from the BS. Forthis purpose, when direct communication is performed, the BS can emptycorresponding DL subframe to allocate the DL subframe as a dedicatedresource for direct communication.

According to the method described above, UE can perform d-DL receptionusing the same frequency region as DL reception without implementingadditional hardware. At this step, UE can store different d-DL timingthan conventional DL timing in the buffer, and enhance the accuracy ofthe d-DL timing through the process of detecting and/or demodulating.

FIG. 17 illustrates a method for acquiring d-DL timing and d-UL timingaccording to another example of the present invention.

In FIG. 17, the meanings of T_(diff-DL), T_(diff-UL) and T_(diff-UE) arethe same as those in FIG. 14.

Referring to FIG. 17, UE 1 and UE 2 perform d-UL transmission and d-DLreception using UL resource based on the UL transmission timing. The ULreception timing can be configured by the BS or acquired from BS.

UE 1 performs d-UL transmission 1 at the first UL subframe based on theUL timing 1, and UE 2 performs d-UL transmission 2 at the second ULsubframe based on the UL timing 2. At this step, predetermined offsetcan be applied to the UL timing, or d-UL transmission can be performedat part of the subframe (e.g. slot) rather than at the entire subframe.

UE 2 performs d-DL reception 1 at the first UL subframe with time delayT_(diff-UE). UE 1 also performs d-DL reception 2 at the second ULsubframe with time delay of T_(diff-UE). The data received by d-DLreception 1 is the data transmitted by d-UL transmission 1, and the datareceived by d-DL reception 2 is the data transmitted by d-ULtransmission 2. The d-DL reception timing 1 and 2 do not coincide withthe DL reception timing 1 and 2. Therefore, different timing thanconventional DL timing is required for the d-DL reception.

Meanwhile, it is preferable that the UL subframe used for directcommunication does not include UL data transmitted to the BS. For thispurpose, when direct communication is performed, the BS can emptycorresponding UL subframe to allocate the UL subframe as a dedicatedresource for direct communication.

According to the method described above, UE can perform d-ULtransmission at the same frequency region as UL transmission withoutimplementing additional hardware. Powers of the UL transmission and thed-UL transmission can be determined differently depending on thesituation.

Meanwhile, communication between UEs in direct communication system canbe affected by the interference caused by the communication betweenother UEs and the BS. This interference can be represented asinter-symbol interference, inter-slot interference or inter-subframeinterference in time domain, and as inter-subcarrier interference infrequency domain. In the description below, inter-symbol interferencerefers to inter-slot interference and inter-subframe interferencecomprehensively.

Inter-symbol interference means the interference due to misalignment ofthe signals received from multiple UEs in time domain, which isgenerated by the symbols received by other UEs in the fast Fouriertransform (FFT) window for the OFDMA symbol (or SC-FDMA symbol, DFT-SOFDMA symbol) received from specific UEs.

Inter-subcarrier interference means that received signals are notaligned in time domain, and that phase discontinuity is generated in FFTwindow for OFDMA symbol (or SC-FDMA symbol, DFT-S OFDMA symbol) andsubcarrier orthogonality is hindered.

In the description below, the symbol can mean OFDMA symbol, SC-FDMAsymbol and DFT-S OFDMA symbol altogether. In other words, the presentinvention is not limited by the access scheme and can be applied tocommunication systems of various access schemes.

FIGS. 18-21 illustrate the interferences that can occur in the examplesof FIGS. 14-17.

FIG. 18 illustrates the interference that can occur in the example ofFIG. 14.

The d-UL transmission at the first DL subframe by UE 1 does not coincidewith the first DL subframe between UE 2 and the BS in timing. Also, thed-UL transmission at the second DL subframe by UE 2 does not coincidewith the second DL subframe between UE 1 and the BS in timing. Thismisalignment can cause inter-symbol interference and inter-subcarrierinterference.

FIG. 19 illustrates the interference that can occur in the example ofFIG. 15.

The d-UL transmission at the first UL subframe by UE 1 does not coincidewith the first UL subframe between UE 2 and the BS in timing. Also, thed-UL transmission at the second UL subframe by UE 2 does not coincidewith the second UL subframe between UE 1 and the BS in timing. Thismisalignment can cause inter-symbol interference and inter-subcarrierinterference.

FIG. 20 illustrates the interference that can occur in the example ofFIG. 16.

The d-UL transmission at the first DL subframe by UE 1 does not coincidewith the first DL subframe between UE 2 and the BS in timing. Also, thed-UL transmission at the second DL subframe by UE 2 does not coincidewith the second DL subframe between UE 1 and the BS in timing. Thismisalignment can cause inter-symbol interference and inter-subcarrierinterference.

FIG. 21 illustrates the interference that can occur in the example ofFIG. 17.

The d-UL transmission at the first UL subframe by UE 1 does not coincidewith the first UL subframe between UE 2 and the BS in timing. Also, thed-UL transmission at the second UL subframe by UE 2 does not coincidewith the second UL subframe between UE 1 and the BS in timing. Thismisalignment can cause inter-symbol interference and inter-subcarrierinterference.

One of the following methods can be applied in order to prevent theseinter-symbol interference and/or inter-subcarrier interference.

<Method 1> Using N Guard Symbols

In one example of the present invention, predetermined N symbols causinginter-symbol interference can be used as guard symbols. Guard symbolsmean the symbols which have no actually transmitted signal. In otherwords, UE can use N guard symbols for direct communication, and performd-UL transmission on other symbols through encoding process such aschannel coding.

Since inter-symbol interference is mainly caused by propagation delayaccording to the distance between UEs, the degree of interference canvary depending on the distance between UEs. When the distance betweenUEs is 1 km, for example, propagation delay T_(diff-)UE is 3.3356 uswhich is much smaller than 1-OFDMA symbol duration, and it is sufficientto use only one OFDMA symbol as a guard symbol.

FIGS. 22-24 illustrate a method for using a guard symbol according toone example of the present invention.

UE can, as in FIG. 22, use the first OFDMA symbol as a guard symbol toprevent inter-symbol interference when performing d-UL transmission.Alternatively, UE can, as in FIG. 23, use the last OFDMA symbol as aguard symbol, or, as in FIG. 24, use the first OFDMA symbol and the lastOFDMA symbol as guard symbols. At this step, the first and last OFDMAsymbols mean both ends of the subcarrier of the radio resource allocatedfor d-UL transmission of a specific UE (or UE-group).

<Method 2> Puncturing N Symbols

In one example of the present invention, predetermined N symbols causinginter-symbol interference can be punctured.

As described above in detail, inter-symbol interference is mainly causedby propagation delay according to the distance between UEs, and thedegree of interference can vary depending on the distance between UEs.When the distance between UEs is 1 km, for example, propagation delay,T_(diff-UE), is 3.3356 us which is much smaller than 1-OFDMA symbolduration, and it is sufficient to puncture only one OFDMA symbol.

UE can puncture the symbol corresponding to the guard symbol in FIGS.22-24 to prevent inter-symbol interference when performing d-ULtransmission. For example, UE can puncture the first OFDMA symbol as inFIG. 22. Alternatively, UE can, as in FIG. 23, puncture the last OFDMAsymbol, or, as in FIG. 24, the first and last OFDMA symbols.

<Method 2-1> Using a Sounding Subframe

In 3GPP LTE, UE punctures the last OFDMA symbol of the sounding subframein order to prevent the interference with sounding transmission by otherUE when performing UL transmission at the subframe designated as thecell-specific sounding subframe or UE-specific sounding subframe. FIG.25 illustrates one example of implicitly puncturing the last symbol of asounding subframe.

Therefore, when d-UL transmission for direct communication is carriedout on UL carrier, direct communication can be specified to be performedin the sounding subframe. According to this method, UEs performingdirect communication may not define punctured symbol separately norperform the signaling. FIG. 26 illustrates one example of a soundingsubframe for direct communication.

<Method 3> Using a Subframe with an Extended CP

Is 3GPP LTE, two types of CPs exist, a normal CP (Δf=15 kHz) and anextended CP (Δf=7.5 kHz). A normal CP is generally used and has thelength of 5.21 us or 4.69, and an extended CP is used for broadcastingsuch as MBSFN and has the length of 16.67 us.

In one example of the present invention, the subframe with the normal CPcan be configured for the communication with the BS, but the subframewith the extended CP can be configured for direct communication betweenUEs, in order to prevent inter-symbol interference. FIG. 27 illustratesone example of using a subframe with an extended CP for directcommunication.

<Method 3-1> the Case where d-UL Transmission is Performed on DL Carrier

In 3GPP LTE, MBSFN subframe is configured throughMBSFN-subframeConfigList at SystemInformationBlockType2 by radioresource control (RRC) layer. The MBSFN subframe may refer to theSection 5.8 and 6.3.7 of the 3GPP TS 36.311 V10.4.0 (2012-12).

In the case where d-UL transmission is performed on the DL carrier, asubset of MBSFN subframes can be configured as the subframe for directcommunication. FIG. 28 illustrates one example of a configuration of asubset of MBSFN subframes as a subframe for direct communication. Atthis step, the MBSFB subframe configured as the subframe for directcommunication should use extended CP, although non-MBSFN subfame can usenormal CP or extended CP.

<Method 4> Applying Time Offset

The methods 1-3 described above are especially effective in removing theinterference of following subframes. In one example of the presentinvention, time offset can be applied to the subframe for directcommunication in order to remove interferences of preceding subframesmore effectively. FIG. 29 illustrates one example of applying timeoffset to a subframe for direct communication.

Referring to FIG. 29, transmission can be delayed by predetermined orsignaled time offset T_(offset) so that the subframe for directcommunication and the subframe for communication with the BS are notoverlapped. For example, time offset can be determined as theunidirectional propagation delay, bidirectional propagation delay orhalf of one symbol duration, and d-UL transmission can be performed withthe delay of the corresponding time offset.

FIG. 30 illustrates a method for data transmission according to oneexample of the present invention.

A first wireless device acquires uplink transmission timing or downlinkreception timing from the BS (S3010). The downlink reception timing canbe acquired by using the DL synchronization signal, and the uplinktransmission timing can be acquired based on the downlink receptiontiming.

The first wireless device, then, determines uplink transmission timingfor direct communication with a second wireless device based on theuplink transmission timing or downlink reception timing (S3020). Theuplink transmission timing for direct communication can coincide withthe uplink transmission timing or downlink reception timing, or thevalue of time offset can be applied.

The first wireless device transmits direct communication uplink data tothe second wireless device at the uplink transmission timing for thedirect communication (S3030). At this step, to perform transmission ofthe direct communication uplink data, the first wireless device can usethe subframe on the uplink resource for transmission to the BS, or thesubframe on the downlink resource for reception from BS. The subframecan include at least one guard symbol, or at least one punctured symbol.Also, the subframe can be a sounding subframe or an MBSFN subframe usingextended cyclic prefix (CP).

FIG. 31 illustrates a wireless communication system in which anembodiment of the present invention is implemented.

A base station 50 comprises a processor 51, a memory 52, and an RF(Radio Frequency) unit 53. The memory 52, being connected to theprocessor 51, stores various pieces of information needed for operatingthe processor 51. The RF unit 53, being connected to the processor 51,transmits and/or receives radio signals. The processor 51 implementsproposed functions, procedures, and/or methods. Operation of the basestation in the embodiment described above can be realized by theprocessor 51.

A UE 60 comprises a processor 61, a memory 62, and an RF unit 63. Thememory 62, being connected to the processor 61, stores various pieces ofinformation needed for operating the processor 61. The RF unit 63, beingconnected to the processor 61, transmits and/or receives radio signals.The processor 61 implements proposed functions, procedures, and/ormethods. Operation of the UE in the embodiment described above can berealized by the processor 61.

The processor may include Application-Specific Integrated Circuits(ASICs), other chipsets, logic circuits, and/or data processors. Thememory may include Read-Only Memory (ROM), Random Access Memory (RAM),flash memory, memory cards, storage media and/or other storage devices.The RF unit may include a baseband circuit for processing a radiosignal. When the above-described embodiment is implemented in software,the above-described scheme may be implemented using a module (process orfunction) which performs the above function. The module may be stored inthe memory and executed by the processor. The memory may be disposed tothe processor internally or externally and connected to the processorusing a variety of well-known means.

In the above description of direct communication, the subframe fordirect communication does not exclude the conventional communicationbetween a BS and a UE. In other words, the subframe for directcommunication can be used either for direct communication only, or forthe conventional communication between a BS and a UE. Also, partialregion (or resource block) of specific subframe can be used for directcommunication and other region for the conventional communicationbetween a BS and a UE.

Meanwhile, the above described methods can be applied to the entireprocedures of the direct communication, or only to the step of initialaccess/search. The step of initial access/search can include ULsynchronization procedure such as random access procedure of 3GPP LTE,or establishing a link by searching neighbor UEs. The methods describedabove can be applied to all UEs or only to the UEs instructed by thebase station.

Also, the above described methods can be modified to be applied toacquisition of synchronization between multiple BSs and one UE. Forexample, the UE can acquire synchronization with other non-serving BSsbased on the synchronization with the serving BS. Also, the methods canbe modified to be applied to acquire synchronization of other UEs basedon the synchronization between one UE and a BS.

What is claimed is:
 1. A method for transmitting data between wirelessdevices in a wireless communication system, comprising: acquiring, by afirst wireless device, downlink reception timing with a base station;determining, by the first wireless device, transmission timing fordirect communication with a second wireless device based on the downlinkreception timing with the base station; and transmitting, by the firstwireless device, data for the direct communication to the secondwireless device at the transmission timing for the direct communication.2. The method of claim 1, wherein the data for the direct communicationis transmitted via a subframe on an uplink resource used for acommunication with the base station.
 3. The method of claim 1, whereinthe data for the direct communication is transmitted via a subframe on adownlink resource used for a communication with the base station.
 4. Themethod of claim 3, wherein the data for the direct communication istransmitted using a part of the subframe on the downlink resource. 5.The method of claim 3, wherein the subframe on the downlink resourceincludes at least one guard symbol.
 6. The method of claim 3, whereinthe subframe on the downlink resource includes at least one puncturedsymbol.
 7. The method of claim 6, wherein the subframe on the downlinkresource is a sounding subframe.
 8. The method of claim 3, wherein thesubframe on the downlink resource uses extended cyclic prefix (CP). 9.The method of claim 8, wherein the subframe on the downlink resource isa multimedia broadcast and multicast single frequency network (MBSFN)subframe.
 10. A method for transmitting data between wireless devices ina wireless communication system, comprising: acquiring, by a firstwireless device, uplink transmission timing with a base station;determining, by the first wireless device, transmission timing fordirect communication with a second wireless device based on the uplinktransmission timing with the base station; and transmitting, by thefirst wireless device, data for the direct communication to the secondwireless device at the transmission timing for the direct communication.11. The method of claim 10, wherein the data for the directcommunication is transmitted via a subframe on an uplink resource usedfor a communication with the base station.
 12. A wireless device in awireless communication system, comprising: a RF (Radio Frequency) unittransmitting and receiving radio signals; and a processor connected tothe RF unit, configured to acquire downlink reception timing with a basestation; determine transmission timing for direct communication with aneighbor wireless device based on the downlink reception timing with thebase station; and transmit data for the direct communication to theneighbor wireless device at the transmission timing for the directcommunication.
 13. The wireless device of claim 12, wherein the data forthe direct communication is transmitted via a subframe on an uplinkresource used for a communication with the base station.
 14. Thewireless device of claim 12, wherein the data for the directcommunication is transmitted via a subframe on a downlink resource usedfor a communication with the base station.
 15. The wireless device ofclaim 14, wherein the subframe on the downlink resource includes atleast one guard symbol.
 16. The wireless device of claim 14, wherein thesubframe on the downlink resource includes at least one puncturedsymbol.
 17. The wireless device of claim 16, wherein the subframe on thedownlink resource is a sounding subframe.
 18. The wireless device ofclaim 14, wherein the subframe on the downlink resource uses extendedcyclic prefix (CP).
 19. The wireless device of claim 18, wherein thesubframe on the downlink resource is a multimedia broadcast andmulticast single frequency network (MBSFN) subframe.